pipe.lua

--- cat 'foo' | uppercase | output

function output(co)
  while 1 do
    local ok, val = coroutine.resume(co)
    if ok then
      print(val)
    else
      return
    end
  end
end

function uppercase(co)
  return coroutine.create(function()
    while 1 do
      local ok, val = coroutine.resume(co)
      if ok then
        coroutine.yield(val:upper())
      else
        return
      end
    end
  end)
end

function cat()
  return coroutine.create(function()
    coroutine.yield('tobi');
    coroutine.yield('loki');
    coroutine.yield('jane');
    coroutine.yield('luna');
  end)
end

output(uppercase(cat()))